The present invention relates to a controller or control system of a compression ignition engine. Particularly, the present invention relates to the controller of the compression ignition engine for preventing any deterioration in exhaust gas purification at a transition between spark ignition operation and compression ignition operation and during the compression ignition.
The conventional lean-burn engines can improve fuel economy as the result of the effect of pumping loss decrease, and the like.
However, there is a problem that NOx during the lean operation cannot be purified with three way catalyst. Therefore, the technology of a compression ignition engine which can suppress the NOx of engine out to several ppms in the lean mixture condition is proposed as one of the solutions of this problem (for instance, see Japanese Patent Application Laid-Open No. Sho 62-15722 Official Gazette, etc.). The compression ignition engine which has been described in the Official Gazette, by charging a pre-mixed air/fuel mixture for self-ignition, enables combustion at a combustion temperature lower than the temperature 1800xc2x0 K. at which NOx generates (FIG. 1).
However, it should be difficult to achieve the compression ignition at the time of engine starting, at the time of high load range of engine operation or the like and it is needed to switch the compression ignition range of engine operation, and the spark ignition range by a sparking plug depending upon the operation range or the state of the engine, as pointed out also with the Official Gazette (FIG. 2). The technology of the controller of the compression ignition engine which achieves this has been proposed (for instance, see Japanese Patent Application Laid-Open No. Hei 11-6435 Official Gazette). The compression ignition engine which has been described in this Official Gazette detects ignition timing based on an ionic current within a combustion chamber. Abnormal combustion is prevented by switching the combustion when a difference between the detected ignition timing and the spark ignition timing of a sparking plug becomes lower than a predetermined value. In another way, NOx under exhaust is detected, and the combustion is switched based on the change in the density of NOx. This technology prevents the combustion from becoming unstable (as proposed technology, see Japanese Patent Application Laid-Open No. Hei 11-336600). At this time, the deterioration in exhaust gas purification due to the miss-firing can be prevented by appropriately judging the timing of the switching.
By the way, the catalyst which purifies the exhaust is not considered even though the conventional technology can prevent the deterioration in exhaust gas purification due to the abnormal combustion possibly caused in transition between the compression ignition and the spark ignition. Therefore, even in the state that the exhaust cannot be purified with the catalyst, the switching for the combustion is carried out, and it occurs a possibility to deprave the exhaust discharged in atmosphere. Namely, if the air/fuel ratio becomes excessively lean condition at the time of switching when the three way catalyst is provided in an exhaust pipe, NOx is exhausted as it is without being purified. On the contrary, if the air/fuel ratio becomes excessively rich condition, HC and CO are exhausted without being purified.
Moreover, when the compression ignition is carried out in a lean air/fuel ratio, NOx cannot be purified with the three way catalyst. Therefore, it necessitates any means by which NOx exhausted from the engine at this time is prevented from deteriorating.
That is, the inventors of the present invention have found that it is possible to monitor the fact that the exhaust gas is purified with the catalyst at the switching transition between the spark ignition mode operation and the compression ignition mode operation by using a three way catalyst and an NOx detector located at the downstream side of the three way catalyst, and it is possible to monitor the fact that the combustion aggravates also at the time of the compression ignition mode operation. They found that on the basis of this knowledge a system which achieves this can be provided with a minimum system structure. The above-mentioned prior art techniques do not have at all special consideration about this.
The present invention has been worked out in view of the problems set forth above. One object of the present invention is to provide a control system for a compression injection engine which can prevent deterioration in exhaust purification at the time of the switching of combustion between spark ignition and compression ignition, at the time of the spark ignition combustion and the compression injection combustion, and can diagnose any deterioration of an NO x detector and a three way catalyst.
In order to accomplish the above-mentioned object, a control system for a compression ignition engine comprises:
a catalytic converter installed within an exhaust passage of the compression ignition engine for compressing and igniting a pre-mixture of a fuel and air;
an air/fuel ratio detector for detecting an air/fuel ratio at upstream side of the catalytic converter; and
an NOx detector for detecting NOx at downstream side of the catalytic converter.
The control system for the compression ignition engine according to the present invention, constructed as set forth above, is provided with the NOx detector located downstream side of the catalytic converter. Since engine control is performed on the basis of the NOx detection value, a minimum system of the compression ignition engine can be constructed. Furthermore, with the construction set forth above, prevention of deterioration of exhaust gas purification performance can be achieved upon switching of combustion mode between spark ignition and compression ignition in addition to combustion in the spark ignition mode and compression ignition mode, and prevention of deterioration of exhaust gas purification performance by diagnosis of the NOx sensor, the catalytic converter and so forth.
On the other hand, the particular mode of implementation of the control system of the compression ignition engine according to the present invention may use an output signal of the NOx detector for preventing deterioration of exhaust gas purification performance. In the alternative, the control system may prevent deterioration of exhaust gas purification performance upon switching transition of combustion mode between spark ignition and compression ignition, during spark ignition mode combustion and during compression ignition mode combustion.
In another aspect of the invention, the control system may further comprise:
combustion mode switching means for switching between a spark ignition mode and compression ignition mode;
exhaust deterioration factor predicting means for predicting a factor causing deterioration of exhaust gas purification performance on the basis of an NOx detection value upon switching transition of the combustion mode; and
switching learning means for leaning control amount and control object of the combustion mode switching means on the basis of the factor causing deterioration of exhaust gas purification performance. More particularly, the switching learning means learns a spark ignition timing, a throttle valve control amount, a fuel injection amount, a fuel cut-off period before switching to the compression ignition mode combustion, one of a target intake air flow rate and a target EGR amount or an engine operational range for switching combustion mode between the spark ignition mode combustion and the compression ignition mode combustion.
In a further aspect of the present invention, the control system performs fuel injection between an expansion stroke and an exhaust stroke in the compression ignition mode combustion. The control system may comprise:
air/fuel ratio control means for controlling an output of the air/fuel ratio detector toward a target air/fuel ratio;
a target air/fuel ratio calculating means for calculating the target air/fuel ratio for optimizing purification ratio of the catalytic converter,
the target air/fuel ratio calculating means calculates the target air/fuel ratio on the basis of an output signal of the NOx detector. The control system may temporarily sets the target air/fuel ratio rich side when the NOx detection value exceeds a predetermined value. The control system may comprise NOx detector diagnosis means for diagnosing abnormality of the NOx detector for controlling the air/fuel ratio so that an output of the air-fuel ratio detector becomes stoichiometric value. The control system may further comprise combustion deterioration detecting means for detecting deterioration of combustion on the basis of an NOx detection value in the compression ignition mode combustion. The combustion deterioration detecting means may detect deterioration of combustion in the compression ignition mode combustion when an output of the air/fuel ratio detector indicates a lean mixture condition. The control system may further comprise combustion improvement control means for controlling improvement of combustion in the compression ignition mode when deterioration of combustion is detected by the combustion deterioration detecting means. The control system may control the air/fuel ratio so that the output of the air/fuel ratio detector becomes stoichiometric value when deterioration of combustion is detected by the exhaust deterioration detecting means after implementation of the combustion improvement control means. The control system may further comprise air/fuel ratio detector diagnosing means for detecting abnormality of the air/fuel ratio detector, for correcting a fuel injection amount on the basis of the output signal of the NOx detector when abnormality of the air/fuel ratio detector is detected. The control system thus prevents deterioration of exhaust gas purification performance by detecting fatigue of at least one of the NOx detector and the catalytic converter.
On the other hand, the control system may comprise fuel cut-off means for interrupting fuel injection during deceleration, and fatigue of NOx detector may be detected on the basis of the output signal of the NOx detector during fuel cut-off. The control system comprises fuel recovery means for recovering fuel injection after fuel cut-off, abnormality of the NOx detector may be judged when a difference the NOx detection value during fuel recovery and the NOx detection value during fuel cut-off is less than or equal to a predetermined value. The control system may further comprise air/fuel ratio control means for controlling air/fuel ratio so that an output of the air/fuel ratio detector becomes close to a target air/fuel ratio, for diagnosis of fatigue of the catalytic converter on the basis of the output signal of the NOx detector when the target air/fuel ratio is set at stoichiometric value.
In addition, an automotive vehicle has a control system for a compression ignition engine constructed as set forth above.